Currently, the standard method used is optical transmissometry, and the quantity measured is opacity (Op), defined as the fraction of incident light which is lost in transmission through an optical medium.
In principle, all that is needed is a light source mounted on one end of the optical path and a detector mounted at the other. In practice, a more sophisticated arrangement is needed to compensate for variations in source intensity, detector response, etc. which would otherwise appear as changes in the opacity measurement.
A preferred form of opacity measurement system known in the prior art is illustrated in FIG. 1 of the drawings hereto, which shows a double-pass system, with a transceiver mounted on one side of the path and a passive reflector on the other side. This has the advantage that it permits an automatic calibration check to be performed by means of a reflector intermittently placed in front of the transceiver.
To provide meaningful data, gain compensation is required to deal with the temperature variations and ageing electronic components, and two prior art methods for measuring opacity with gain compensation are known. Both rely on continuously moving reflectors. The first method is used in the Erwin Sick RM41 instrument. This has a single detector which alternately views the distant retroreflector and internal, partially reflecting chopper. This does away with the need for the second detector and permits an external zero reflector to be used to check the instrument calibration. The method works well provided high quality (i.e. expensive) bearings are used for the chopper motor. The second method, employed in the United Sciences Model 500 instrument, combines the zero reflector with the partially reflecting chopper, thereby simplifying the instrument, but only at the cost of losing the independent zero check. This method still suffers from the drawback that it requires a continuously rotating reflector.